Interferon is a naturally-occurring antiviral protein produced by most cells upon stimulation by virus infection or other substances recognized as interferon "inducers". The potential of interferon as a broad spectrum antiviral and/or anti-tumor agent has long been recognized.
Double stranded RNA (dsRNA), whether of synthetic, e.g., polyriboinosinic-polyribocytidilic acid (poly I:poly C), or natural origin, is recognized as an excellent inducer of interferon. Unfortunately, the usefulness of natural or synthetic dsRNA is greatly limited due to its rapid degradation by nucleases present in the sera of various animal species, particularly primates. More significantly, the enzymatic degradation of dsRNA is especially apparent in human serum; see, for example, R. J. Douthart and S. G. Burgett, Biochem. and Biophys. Res. Comm. 84 (1978), 809-815.
It is essential, therefore, if dsRNA is to be useful as an inducer of interferon in primates, in particular, humans, that its rapid enzymatic degradation be prevented or substantially diminished.
Attempts have been made to promote the interferon-inducing activity of dsRNA, for example, by stabilizing it against enzymatic degradation by a variety of approaches.
W. E. Houston, C. L. Crabbs, E. L. Stephen, and H. B. Levy, Infection and Immunity 14, (1976), 318-319, report stabilization of poly I:poly C against enzymatic degradation using poly-L-lysine and carboxymethyl-cellulose.
A. Billiau, C. E. Buckler, F. Dianzani, C. Uhlendorf, and S. Baron, Ann. N.Y. Acad. Sci., 173 (1970) 657-667, report that stimulation of the interferon mechanism in tissue culture by poly I:poly C can be enhanced by addition of substances such as neomycin, streptomycin, diethylaminoethyl dextran (DEAE-dextran), methylated albumin, protamine, histone, and colistin. Neomycin and protamine, each when administered to mice in conjunction with poly I:poly C, were shown to have no effect on the yield of circulating interferon. DEAE-dextran, on the other hand, showed a definite enhancement of the interferon response from separately-administered poly I:poly C.
G. P. Lampson, A. A. Tytell, A. K. Field, M. M. Nemes, and M. R. Hilleman, Proc. Soc. Exp. Biol. and Med., 132, (1969) 212-218, measured the effect of several polyamines used in conjunction with poly I:poly C. Of ten polyamines tested, five demonstrated some effect toward the stabilization of poly I:poly C in the presence of RNase. Most of the testing was carried out using neomycin with the stated conclusion that the "effect of neomycin was entirely limited to in vitro activity in one particular kind of cell and showed no potential for practical utilization in human and animal application either in potentiation of poly I:C activity or in reduction of its toxicity." (page 217).
J. J. Nordlund, S. M. Wolff, and H. B. Levy, Proc. Soc. Exp. Biol. and Med., 133, (1970) 439-444, suggest that human plasma is capable of rapid enzymatic degradation of poly I:poly C in contrast, e.g., to rabbit serum. They further report the elimination by neomycin of the destructive capacity of dilute (15%) human plasma.
A new complex of natural or synthetic dsRNA recently has been discovered. This complex, a combination of dsRNA and tobramycin has been shown to have the highly advantageous properties of (1) ability to induce interferon; (2) stability in the presence of human plasma containing dsRNase; (3) ability to activate peritoneal macrophages; and (4) enhancement of the activity of dsRNA against tumor systems, including highly refractory tumors such as the Madison lung tumor, even in species having low levels of dsRNase in which there is no demonstrable difference in peak interferon titers produced by dsRNA alone and the tobramycin:dsRNA complex.